A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the hydrogen- or alcohol-fuelled proton exchange membrane fuel cell (PEMFC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.
The principal component of the PEMFC is the membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanoparticles (such as metal blacks or other unsupported particulate metal powders) or can be deposited as even higher surface area particles onto a conductive carbon substrate or other conductive material (a supported catalyst).
Conventionally, the MEA can be constructed by a number of methods outlined hereinafter:
(i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. A gas diffusion electrode is placed on each side of an ion-conducting membrane and laminated together to form the five-layer MEA;
(ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, a gas diffusion layer is applied to each face of the catalyst coated ion-conducting membrane.
(iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs; removing products; providing electrical connections; and providing physical support.
Typically, the gas diffusion layers are formed from carbon fibre based gas diffusion substrates having a layer of particulate material (a microporous or base layer), such as carbon black and polytetrafluoroethylene (PTFE), on one face of the gas diffusion substrate, such that when formed into a MEA, the microporous layer contacts the electrocatalyst layer.
It is an essential requirement of a gas diffusion substrate that it is porous, electrically conductive and mechanically stable. The gas diffusion substrates most widely commercialised to date are made from carbonised polyacrylonitrile (PAN) fibres using a wet-laid or dry-laid process to produce a non-woven web of carbon fibres. The non-woven web is generally impregnated with an organic resin binder material (e.g. a phenolic resin) that is subsequently carbonised/graphitised when heat treated to a high temperature of in excess of 2000° C. Gas diffusion substrates manufactured using this high temperature process possess the required conductivity, stability and mechanical strength, but the process is extremely energy intensive and contributes significantly to the cost of these substrates.
Gas diffusion substrates similar to those described above, but using an intermediate-temperature process (around 1500° C. to 2000° C.) for the carbonisation of the organic binder material have been prepared; the process for preparing such substrates is less energy intensive than the higher temperature process and thus the cost of the substrates is less. Furthermore, such substrates are less rigid and can be prepared as a roll-good product.
Gas diffusion substrates that do not require a high temperature carbonisation or graphitisation step have also been proposed by incorporating a dispersion of a hydrophobic polymer and carbon black or graphitic particles into a non-woven carbon fibre network. Although these substrates are a lower cost option, the conductivity of such substrates may not be sufficient for some applications.